Multi-layer coating systems were developed to satisfy a need for improved aesthetics of the coated substrate, especially automobiles. By applying a relatively thin pigmented layer, termed the "basecoat", and then overcoating with a thicker, unpigmented layer as a topcoat, termed the "clearcoat" it was possible to achieve a glossy coating with good appearance, weatherability, anti-solvent and chemical resistances and anti-discoloring resistances.
In order to obtain the desired appearance of the coating it is necessary to add a rheology control agent like fumed silica, bentone clays or gel particles to the topcoat composition.
These systems are described for example in U.S. Pat. No's. 4,493,914 NAD microgel, 4,377,661 Crosslinked microgel particles, 4,455,331 silica, 4,526,910 silica, 4,558,075 Bentone Clay, and 4,620,994 Bentone Clay.
In addition, in a multi-layer coating system it is necessary that the basecoat have "strike in" resistance. By "strike in" resistance is meant the ability of the basecoat to resist attack by the solvents in the topcoat composition. The strike in is a problem because the automobile manufacturers generally wish to apply the topcoat composition in a color plus clear system by a "wet-on-wet" technique. By this is meant that the basecoat composition is applied to the substrate followed by the topcoat composition and then a single baking step is utilized to cure the composite coating. The topcoat "striking in" to the basecoat is particularly undesirable since it adversely affects alignment of the metallic pigment. In highly pigmented colors the non-metallic pigment particles can provide the physical barrier to prevent strike in. But in silver and light metallic colors there is insufficient pigment to prevent strike in. The rheology control agent or a filler pigment must be used to prevent the strike in.
The U.S. Pat. No. 4,728,543 discloses a clear topcoat composition comprising a film-forming acrylic polymer and internally cross-linked polymer microgel particles to overcome the above-mentioned disadvantages.
The object of the present invention was to provide a coating composition as a clearcoat which does not need the addition of a conventional rheology control agent or the use of microgel particles and which shows improved appearance and sag resistance and which overcomes the striking in.
Another object of the invention was a method of coating an article and the coated article.